1. Field of the Invention
This invention relates to a liquid crystal display. More particularly, the invention relates to a liquid crystal display and a driving method thereof that are adaptive to compensate for a brightness deviation among red, green and blue colors.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls light transmittance of a liquid crystal having a dielectric anisotropy using an electric field to display a picture. To this end, the LCD includes a liquid crystal display panel having a pixel matrix, and a driving circuit for driving the liquid crystal display panel.
Specifically, as shown in FIG. 1, the LCD includes a liquid crystal display panel 2 having a pixel matrix, a gate driver 4 for driving gate lines GL0 to GLn of the liquid crystal display panel 2, a data driver 6 for driving data lines DL1 to DLm of the liquid crystal display panel 2, and a timing controller 8 for controlling driver timing of the gate driver 4 and the data driver 6.
The liquid crystal display panel 2 includes a pixel matrix consisting of sub-pixels 10 defined by crossings of the gate lines GL and the data lines DL. Each of the sub-pixels 10 includes a liquid crystal cell Clc for controlling a light transmission amount in accordance with a pixel signal, and a thin film transistor TFT for driving the liquid crystal cell Clc.
The thin film transistor TFT is turned on when a scanning signal, that is, a gate high voltage VGH from the gate line GL, is applied to provide a pixel signal from the data line DL to the liquid crystal cell Clc. Further, the thin film transistor TFT is turned off when a gate low voltage VGL from the gate line GL is applied, and a pixel signal is charged in the liquid crystal cell Clc.
The liquid crystal cell Clc can be expressed equivalently as a capacitor, and consists of a common electrode opposite a pixel electrode having a liquid crystal therebetween connected to the thin film transistor TFT. The liquid crystal cell Clc further includes a storage capacitor Cst that stores the charged pixel signal until a next pixel signal is charged. The liquid crystal cell Clc changes an alignment state of the liquid crystal having a dielectric anisotropy in accordance with the pixel signal charged through the thin film transistor TFT.
The gate driver 4 shifts a gate start pulse GSP from the timing controller 8 in response to a gate shift clock GSC to thereby sequentially apply a scanning pulse having the gate high voltage VGH to the gate lines GL1 to GLm. The gate driver 4 also supplies a gate low voltage VGL to the gate lines GL in the remaining interval at which a scanning pulse having the gate high voltage VGH is not applied. Further, the gate driver 4 controls a pulse width of the scanning pulse in response to a gate output enable signal GOE from the timing controller 8. Such a gate driver 4 includes a plurality of gate driving integrated circuits (IC's) to permit selective driving of the gate lines GL0 to GLn.
The data driver 6 shifts a source start pulse SSP from the timing controller 8 in response to a source shift clock SSC to generate a sampling signal. Further, the data driver 6 latches pixel data RGB input in accordance with the source shift clock SSC in response to the sampling signal and thereafter supplies the latched sampling signal line by line in response to a source output enable signal SOE. Then, the data driver 6 converts the pixel data RGB supplied line by line to analog pixel signals in response to a gamma voltage from a gamma voltage source (not shown) to apply them to the data lines DL1 to DLm. Herein, the data driver 6 determines a polarity of the pixel signal in response to a polarity control signal POL from the timing controller 8 when the pixel data is converted to the pixel signals. Further, the data driver 6 determines a period when the pixel signals are applied to the data lines DL1 to DLm in response to said SOE signal. The data driver 6 includes a plurality of data driving integrated circuits (IC's) for separately driving the data lines DL1 to DLm.
The timing controller 8 generates GSP, GSC, GOE signals, etc. to control the gate driver 4, and SSP, SSC, SOE and POL signals, etc. to control the data driver 6. The timing controller 8 generates control signals such as GSP, GSC, GOE, SSP, SSC, SOE and POL, etc. using a data enable signal DE for informing an effective data interval input from the exterior, a horizontal synchronizing signal Hsync, a vertical synchronizing signal Vsync and a dot clock DCLK for determining a transmission timing of the pixel data RGB.
The related art liquid crystal display device has a red (R) color filter, a green (G) color filter and a blue (B) color filter for each sub-pixel on the upper substrate of the liquid crystal display panel 2, as shown in FIG. 2. Generally, a combination of three sub-pixels 10 provided with such R, G and B color filters used to provide color for one pixel. The R, G and B color filters are formed for each cell area, that is, for each sub-pixel area defined by a black matrix having a matrix type. A common electrode 20 is entirely coated onto the upper substrate having the R, G and B color filters to apply to a common voltage Vcom to the liquid crystal cell Clc. The common voltage Vcom applied to the common electrode 20 keeps a constant direct current voltage as shown in FIG. 3. Further, a pixel signal D, alternating a positive(+) polarity and a negative(−) polarity on the basis of the common voltage Vcom as shown in FIG. 3, is applied, via the data line DL and the thin film transistor TFT, to the liquid crystal cell Clc.
However, in the related art liquid crystal display device, because the R, G and B color filters have a different transmission characteristic, transmittance (T) curves of the R, G and B lights related to a supply voltage V have a different shape as shown in FIG. 4A and FIG. 4B. FIG. 4A and FIG. 4B show transmittance (T) curves of R, G and B lights related to a supply voltage V at each of the R, G and B liquid crystal cells using a liquid crystal having an optically compensated bend (OCB) mode. Herein, a 650 nm waveform means an R light; a 550 nm does a G light; and a 450 nm does a B light. Referring to FIG. 4A and FIG. 4B, it can be seen that, as light transmittances at the R, G and B liquid crystal cells are different from each other, supply voltages V corresponding to minimum brightness of the R, G and B lights at a black level are different from each other. More specifically, as can be seen from FIG. 4B, a supply voltage V corresponding to a minimum brightness of the B light is smaller than 4V, whereas a supply voltage V corresponding to a minimum brightness of the G or R light is larger than 4V. Accordingly, since a brightness of the B light becomes relatively high when supply voltages V corresponding to minimum brightness of the G and B lights are determined by a black level voltage, a color shift phenomenon causing a rise of the black level occurs. Because such a color shift phenomenon reduces a contrast ratio, sharpness of a picture displayed on the liquid crystal display device is deteriorated.